MRI Flashcards
(23 cards)
what creates the nuclear magnetic dipoles necessary for MRI?
Spinnings of atoms creates magnetic dipoles
To create these nuclear magnetic dipole moments, an atom needs to have an odd number of nucleons
(ex: hydrogen w/1 proton & no neutrons)
Preconditions for imaging atomic nuclei
Nuclei must have an odd number of nucleons
Nuclei must be abundant in body
What creates the magnetic field in an MRI
superconductive coils cooled with helium create the homogenous, static B field required
What happens when atomic nuclei enter the static magnetic field
Spins of atomic nuclei are normally oriented randomly
When in a magnetic field, the spins align w/ the applied field either in parallel or antiparallel
Excess along B (in the form of an applied field) results in net magnetization (= the sum of the dipole moments)
Along what axis does precession occur?
atoms precess or wobble due to magnetic momentum of an atom along the Z access
On what does the frequency of precession depend?
Lamor frequency depends on gyromagnetic ratio (for hydrogen = 42.6 MHz/T) and B (magnetic field strength)
The stronger the B field, the faster the precession
How are atoms excited in an MRI?
A radio frequency pulse (RF) is sent w/ a frequency that matches the center frequency of the system
What is the result of an excitation RF pulse?
RF pulse runs perpendicular to B field (switches net magnetization to xy plane) → causes spins to be in phase
The processing spins induce a current in the receiver coil
Which takes longer, T2 relaxation or T1 relaxation?
T1 relaxation is longer
What is T1 relaxation?
relaxation in the longitudinal plane (Z axis)
After RF pulse, the spins are flipped toward the xy plane → spins of the protons transfer the energy to the tissue, then flip back toward the Z-axis -→ magnetization in the z axis is therefore recovered
What is T2 relaxation?
relaxation in xy plane/transverse decay time constant
after excitation, spins all point in the same direction, but over time, inhomogeneities of B field) cause spins to be in different locations w/ different 𝝎 (spins dephase)
What are the gradients produced by the gradient coils?
1) Selective slice excitation (Gss = Gz) → peaks same time as RF pulse
2) Phase encoding gradient (Gpe = Gy) → switched on briefly in anterior-posterior position and when it’s turned off, each proton within the selected slice will spin with the same frequency but with a slightly different phase
3) Frequency encoding gradient (Gro = Gx) → gradient causes variation of precession frequency w/ position, receive sum of signals from spin
Which of the following is/are true:
a) flip angle is energy dependent
b) flip angle does not depend on RF pulse strength
c) flip angle is dependent on pulse duration
d) flip angle is dependent on RF pulse strength and duration
a, c, d
What do pulse sequences control?
RF excitation
Gradient waveforms
Acquisition
Image reconstruction
What are the (general) steps of the spin echo sequence
1) after 90º RF pulse, net magnetization is in xy plane
2) Spins immediately start to de-phase due to T2 relaxation
3) A second RF pulse is given, but this time it’s a 180º pulse
4) 180º pulse causes the spins to re-phase (= a compensation of the static field inhomogeneities)
What are the variations of the spin echo sequence?
Multi-echo sequence
Multi-slicing sequence
Inversion recovery sequence
How does the multi-echo sequence differ from the standard spin echo sequence?
this time a series of seven 180º pulses are given
→ each 180º pulse generates an echo
→ the k space is divided into 7 segments & each echo fills one line in each segment
How does multi-slicing differ from standard spin echo sequence?
As soon as the first repetition is finished, the next repetition is started, but Gss is shifted so that a slice next to the first slice is selected
TR = 540 ms &TE = 30 ms
→ can do 18 slices in one TR
After all slices have been done the first time, go back to first Gss then do second repetition
What does the inversion recovery sequence do?
How is it done?
Increases T1 contrast
Usually long TR and short TE
During the first 180º excitation pulse, the net magnetization is flipped to the Mz axis (no magnetization in xy plane yet)
=> only T1 recovery happening
T1 relaxation allowed to occur for a certain amount of time (=TI)
Basically = a spin echo sequence preceded by an additional 180º pulse
How do gradient echo sequences occur?
1) Slice selected w/ Gss
2) Excitation pulses sent
3) Gro turned on w/ negative polarity
→ polarity switch has the same effect as a 180º RF pulse
4) Spins de-phase
5) Gro switched to positive polarity
6) Spins rephase → are in phase again
7) Singal sampling during Gro
What do gradient echo sequences allow us to measure?
T2* relaxation (sensitive to local fluctuating and static field inhomogeneities)
Compare spin- to gradient-echo sequences
In spin, echo is caused by 180º RF pulse + gradient fields
In gradient, echo is caused by gradient fields
The flip angle in spin echo is 90º while it is variable, but less than 90º in gradient (the smaller the FA, the shorter repetition time TR)
Spin echo is time consuming whereas gradient echo is fast
Spin echo gives T2 contrast while gradient echo gives T2* contrast
Spin echo is insensitive to static field inhomogeneities and sensitive to motion artifacts
Gradient echo is sensitive to static field inhomogeneities but insensitive to motion artifacts
On what does T1 contrast depend?
T1 relaxation
